1. Field of the Invention
The subject invention is directed to capacitive pressure sensor, and more particularly, to high temperature capacitive static/dynamic pressure sensors, such as sensors or microphones for detecting acoustic pressure waves in a gas turbine engine, which employs a diaphragm made from a material exhibiting high thermal strength, thermal stability and oxidation resistance at elevated temperatures.
2. Description of Related Art
Capacitive pressure sensors are well known in the art, as disclosed for example in U.S. Pat. No. 6,148,674, the disclosure of which is herein incorporated by reference in its entirety. However, these devices have limited applicability at elevated temperatures. In particular, since the capacitance of prior art pressure sensors are normally in the picoFarad (pF) range, they are susceptible to stray capacitance and other environmental conditions. This makes it difficult to develop high temperature capacitive pressure sensors for use in harsh environment applications, such as, gas turbine applications. Indeed, there are currently no commercially available capacitive pressure sensors capable of operating above 350° C.
This obstacle has been overcome, in part, by a combination of guard techniques and frequency modulation (FM) capacitive transmitting technology. In order to achieve accurate and reliable gauging, all stray capacitance has to be excluded in the signal pick-off circuit. This can be prevented by a technique called guarding. Guarding is accomplished by surrounding the sensing electrode area with a non-sensing conductor that is kept at the same voltage as the sensing area itself. This technique is also used to guard a tri-axial cable connecting the pressure detector to a signal conditioning circuit. As a result, there is no loss of the input signal, even though the cable may be as long as up to 10 meters.
Another factor that influences capacitive pick-off signal is the resistance of the dielectric material, which decreases as the temperature increases. This makes the direct current (DC) signal difficult to detect at high temperatures. A frequency modulation (FM) capacitive measurement system resolves this problem. Use of FM makes the system sensitive to carrier frequency variations and ignores ESD (Electro Static Discharge) or EM (Electro Magnetic) varying fields.
U.S. Pat. No. 7,258,806 to Ho, the disclosure of which is herein incorporated by reference, discloses a diaphragm for a capacitive microphone device. The diaphragm is made using a first substrate that has a dielectric layer formed thereon. Then a second metallic diaphragm layer is applied to the substrate over the dielectric layer. It is not uncommon to manufacture a diaphragm using layers of materials in order to improve the sensing characteristics of the microphone. However, laminar diaphragms are expensive to manufacture and are not capable of operating in high temperature environments.
A solution to these deficiencies has been to employ an all metal diaphragm to minimize the thermal mismatch between the diaphragm and the substrate. However, one of the drawbacks of a metal diaphragm in a pressure sensor or microphone is temperature hysteresis and pressure hysteresis at high temperature. A useful capacitive pressure sensor having a diaphragm made of Inconel 750 has been manufactured and tested by the present inventors. A temperature cycle test from −55° C. to 500° C. at pressure from 15 psi to 600 psi shows that the temperature hysteresis in such a device is approximately 0.76% and the pressure hysteresis is less approximately 1.3% without any thermal compensation. These values are relatively large compared to a silicon diaphragm, which is usually less than 0.25%. In order to reduce the hysteresis effect, a better high temperature diaphragm material is desired. The subject invention is directed to a pressure probe having a diaphragm made of a material that overcomes the drawbacks of the prior art and constructed to provide a more accurate measurement of the pressure (dynamic or static) at high temperatures.